The present invention relates to movable assemblies with fixed ranges of motion, and more particularly to magnetically biasing such an assembly toward one extreme of its range.
Some conventional magnetic latches exert significant attraction only at a region near one extreme of the stroke of a movable assembly. See, for example, FIG. 3 of U.S. Pat. No. 5,782,512 (xe2x80x9cMagnetic Field Latch Assemblyxe2x80x9d) issued Jul. 21, 1998 to Cargnoni. Magnetic latches sometimes require the use of an additional magnet solely to serve that purpose. See U.S. Pat. No. 5,170,300 (xe2x80x9cMagnetic Parking Device for Disk Drivexe2x80x9d) issued Dec. 8, 1992 to Stefansky. One problem that exists in many latch designs of this type is that the latching force is only exerted over a limited range which could not be extended economically.
More recently, longer-range biasing schemes have been developed that allow a magnetic element (of a magnetically permeable material) to pass into a space of maximum flux density, between the centers of a pair of flat (plate-shaped) magnets. At one extreme of the stroke, only a small portion of the element is between the magnets. At the opposite extreme, the latched position, almost all of the element is between the flat magnets. Throughout the stroke, these devices provide an ample biasing force by having magnetic material just outside of the area between the magnets (i.e. overlapping the fringe). A significant shortcoming of this approach, however, is that magnetic elements traverse regions of high and variable flux gradients throughout the majority of the movably assembly""s stroke. Therefore, their biasing effects vary in complex ways that are not easily controlled.
The present invention is directed to the problem of improving magnetic biasing control across an extended range of operation.
Devices of the present invention provides a mass of magnetic material that is elongated in its direction of travel, the mass preferably having a length at least 2-5 times greater than its width. The mass may be one contiguous piece of magnetic material or may be a plurality of separate pieces. Increments of the mass each traverse a path that is configured to approach a strong magnetic field, inducing attraction, and then travel substantially parallel to or tangent to an edge, so that the attraction does not greatly influence travel along its path. By controlling which material does and does not affect attraction substantially, the present invention extends and controls the force profile of the movable assembly economically.
Devices of the present invention operate with a stationary assembly including at least one magnet having a major surface of one magnetic pole, the surface generating a strong magnetic field in the region adjacent to the surface. A movable assembly is coupled to the stationary assembly to provide one degree of freedom with respect to a stationary assembly. The movable assembly travels within a fixed curvilinear range, mechanisms for which are known in the art. The assemblies are coupled so that, over a first portion of the stroke, magnetic material in the movable assembly travels toward or away from the magnetic field. Over a second portion of the stroke, the same magnetic material travels along an edge of the surface, preferably straddling the edge so that some of the material is directly over the surface. For some increment of the mass of magnetic material, each of these portions preferably extends over at least 10-30% of the stroke.
The magnet may operate singly or in concert with one or more additional magnets to create a gap into which magnetic material of the movable assembly can travel. In either case, certain increments of magnetic material travel along the nearest edge of the primary surface or gap. These increments do not contribute significantly to the attraction acting on the movable assembly, traveling substantially parallel to the isomagnetic lines (isolines). By providing configurations with this property, the present invention reduces the uncertainty that ordinarily necessitates complex modeling and prototyping.
In one embodiment, devices of the present invention can function in two modes. In the first mode, the movable assembly interacts with at least one magnet so that it can latch at one extreme of its stroke. Preferably, magnetic material in the movable assembly overlaps the fringe such that most of the material is outside the fringe throughout the stroke. In the second mode, another magnet is added, the presence of which alters the magnetic field in the path of the magnetic material. In a preferred embodiment, the bias direction is the same throughout the stroke of the movable assembly, irrespective of the presence of the additional magnet.
One method of the present invention for assembling a mechanical system includes latching the movable assembly with as few as one flat magnet. An additional magnet may optionally be added, depending on the desired biasing.
Still another embodiment of the present invention includes a movable assembly with a plurality of discrete elements of magnetic material that are spaced from each other. One subset of the elements can thus have an attraction that dominates the other(s), when the movable assembly is at one end of it stroke. Other features and advantages of the present invention will become apparent upon a review of the following figures and their accompanying description.